This invention relates generally to metal detection apparatus, and more particularly the invention relates to a system which detects signals emitted from a metal object such as an underground pipe.
In the construction and utility maintenance fields, the ability to trace underground metal and underground piping has been a needed requirement for both safety and maintenance purposes. Currently available equipment to do this job uses very low frequency systems for detection and tracing. These frequencies may start at 1 kHz and go as high as 490 kHz.
The ability to detect and trace underground piping is greatly affected by the environment that exists at the time of detection. If the ground is very wet or is made of different combinations of earth, erroneous results may occur. If the underground pipe or metal contains an insulating gasket, this may limit the distance at which the pipe can be traced due to the gasket acting like a very low value capacitor. This capacitor is a high impedance to the very low frequency signals, thus preventing them from jumping across the gasket. In addition, the ability to couple the low frequency energy to the underground metal or pipe becomes very difficult. The extreme low frequency makes necessary a very high voltage for the pipe to emit enough signal to be detected.
In accordance with the invention, the location of a metal object such as an underground pipe is detected by first applying an electrical signal to the object for transmission at a transmission frequency. The transmitted signal from the object is then received with a pair of spaced antennas. The received signals from the antennas are applied through a switch to a FM demodulator receiver. The switch alternately connects the antennas at a switch frequency whereby the FM demodulator produces a phase variable signal at the switch frequency. The amplitude of the phase variable signal is detected whereby a null indicates that the antennas are equal distance from the metal object. The phase of the phase variable signal is compared with the switch signal as a reference whereby the phase indicates relative positioning of the antennas with respect to the metal object.
In a preferred embodiment the transmitter section comprises a radio frequency oscillator and a radio frequency power amplifier. The output of the radio frequency power amplifier is connected to an impedance matching network. This network is used to turn the pipe or metal that is being driven into a loss transmission line radiator. This type of transmission line has the characteristic of matching the transmitter impedance very well. Over its length however it has a relatively high resistance which allows the electrical signal that is coupled to it to be electromagnetically radiated. The pipe or metal acts as a single element, surface effect, low transmission line electromagnetic transducer (antenna) and therefore does not require a signal return.
The receiver section in the preferred embodiment includes a pair of antennas, an antenna selecting switch, a matching network, a radio frequency modulation demodulator receiver, a synchronized clock circuit, an ultra high Q band pass filter, a differential phase detector, and phase and amplitude indicators.
The antennas must be of the same type and are spaced at an odd wavelength increment. The antennas are connected to the selection switch which is driven by a clock signal from the synchronized clock generator. The output from this switch feeds the RF input of the FM demodulator receiver. The detection output of the receiver feeds the band pass filter and the output of the band pass filter feeds one port of the phase detector. The other port of the phase detector is fed from a reference signal generated by the synchronized clock generator. This provides synchronized switching signals for the antenna switch as well as a synchronized reference signal for the phase detector. The output of the band pass filter also feeds an amplitude indicator, while the output of the phase detector feeds a phase difference indicator.
Since the two antennas are identical they have identical pickup patterns. When both antennas are not over and equidistance from the driven pipe, and are being switched back and forth rapidly by the signal from the synchronized clock generator, there is a phase modulation developed at the receive frequency. Phase modulation like frequency modulation can be demodulated by a FM demodulator receiver The output of the FM receiver comes through as a tone with a specific phase in reference to the rate at which the antennas are being switched. As the antennas are moved over the pipe so that the receive distance to both antennas is the same, the tone disappears and a null is achieved. This is displayed on the amplitude indicator. When the switched antenna array is moved either to the left of the pipe or to the right of the pipe, the phase of the resulting FM demodulated signal shifts in respect to the switched antennas. The shift in the detected phase drives the phase indicator.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.